1. Field of the Invention
This invention relates to an elutriation apparatus for the purification and separation of powders of different densities, the apparatus comprising at least two classifying or separating columns having two zones of different section, an inlet for a carrier or transporting fluid, means for the introduction of powder to be classified, an outlet for carrier fluid at the upper part of the column and a recuperation filter through which the carrier fluid passes at the outlet of the column.
2. Discussion of the Background
The manufacture of parts in special alloys by sintering metallic powders has enabled improvement in the mechanical properties and reduction in the cost price of the parts. However, in order to reproduce in a consistent manner, high mechanical qualities from the manufacturing process it is necessary to utilise powders of high purity. The chemical purity and more particularly the absence of non-metallic impurities is of very high importance, particularly in respect of the fatigue strength at elevated temperatures. An important application is the manufacture of rotary parts, for turbines and compressors of jet engines, which are liable to high stresses. The demand for high purity powders, having an adequately defined granulometric spectrum, necessitates production means which are both complex and sensitive.
The article "Superclean Superalloy Powders" by Paul Loewenstein presented at the Powder Metallurgy Superalloy Conference Nov. 18-20, 1980, describes an apparatus enabling determination of the concentration of non-metallic inclusions in a metallurgical powder. The method, developed by Nuclear Metals Inc. (NMI), relies upon the principle of elutriation in water. A predetermined quantity of powder is placed in a glass column comprising at its lower part a screen capable of retaining the powder. Deionized and deaerated water is delivered to the base of the column so that the powder forms a fluidized bed mist of cloud. Water leaves the column at its upper part above the fluidized cloud, through an overflow tube and passes through a filter on which any non-metallic particles are retained. The number and the nature of the recovered particles is determined by a microscope.
This apparatus does not enable the production either continuously or semi-continuously of articles of high purity; and in fact the sole object is the control of the purity of the powder tested.
An elutriation method using air as the carrier or vehicle fluid is described in U.S. Pat. No. 3,457,336 and has as its object the formation of fat particles starting from molten droplets. The elutriation air is admitted at the lower part of the column. An inlet adjacent to the air inlet enables the supply of powder. The air and the powder carried thereby leave at the upper part of the column and pass through a filter. The powder retained is eventually recycled into the inlet. Droplets of molten product are introduced at the upper part of the column and fall into the powder cloud, within which they are cooled. They are evacuated at the lower part of the column in a continuous manner. The air current and the powder serve to cool the droplets and to avoid their aggregation. It is not sought to separate the granulometric mixture of powders, the quantity of droplets being substantial in relation to the amount of powder, but to separate the non-agglomerated powder from the droplets in order to produce at the outlet true granules.
Another apparatus is known from French Pat. No. 2318 681 enabling the separation of resins of different density. The column in which this separation is effected has two zones of different diameter: a lower zone of smaller diameter and an upper zone of larger diameter forming a torus around the upper end of the zone of smaller diameter. The mixture of resins is placed on a screen mounted adjacent to the bottom of the apparatus. The carrier water is delivered through the base and carries the resins which form a cloud of which the lighter grains occupy the higher part of the column. The water flows at a level above that of the upper limit of the cloud. The lighter fractions of the resins separate in the upper torus and the heavier fractions in the lower part of the column.
This apparatus does not permit separation, at least in a semi-continuous manner, the grains of a powder.
For various reasons referred to, these different processes do not enable powders to be separated in a simple manner, and in the case of a liquid, it is necessary, except in instances which relate to resins, to dry the separated powders, which increases the risk of pollution.